The present invention generally relates to a tennis racket and more particularly, to a tennis racket frame intended to improve its repulsion performance, i.e. the flying performance of a ball, which is an important characteristic of a tennis racket.
Recently, in connection with tennis racket frames, freedom for designing has been enlarged due to molding thereof by a fiber reinforced plastic such as a carbon fiber reinforced plastic, with a consequent result of higher performance. By way of example, there has been provided a so-called "large-sized racket" increased in its ball striking area, or "wide body" and with a larger thickness in the ball striking direction, whereby a wider application is achieved.
The fact that tennis rackets of the above described type have been widely employed, shows that in tennis play, except for some limited high class players capable of controlling speed and ball power, as desired, the most important performance required for tennis players in order to play comfortably is the repulsion performance, i.e. the flying performance of the ball after it strikes the strings.
It is self-evident that, during actual play, the respective functions of the ball, the tennis racket frame, and the strings act in concert with to each other in producing the phenomenon which defines the way in which "the ball flies". Although detailed functions in such interactions are not fully clarified, it has been found that conversion of energy at the point of collision between the ball and the racket, into kinetic energy is essential for flying the ball, and for this purpose, the rigidity or stiffness of the racket frame and the position of the sweet area on the ball striking face is very important.
With respect to the rigidity as referred to above, the rigidity in the direction from where the struck ball comes flying, and in the ball striking face outer direction which is the direction wherein the ball is driven back after having collided with the tennis racket, i.e. the rigidity in the direction of thickness of the frame, and the rigidity in the ball striking face inner direction, which intersects at right angles with the above ball striking face outer direction, i.e. the rigidity in the direction of the main strings and that in the direction of the cross strings, are important.
For the rigidity of the racket frame as described above, various proposals have been made up to the present, each of which, however, is limited to an improvement only with respect to either the rigidity in the ball striking face outer direction or in the ball striking face inner direction. For example, in a conventional racket frame disclosed in Japanese Patent Laid-Open Publication Tokkaisho No. 62-231682, it is intended to increase the rigidity in the ball striking face inner direction for improving the repulsion performance by positionally varying the geometrical moment of inertia of the frame or by partially reinforcing the fibers.
However, in order to improve the repulsion performance by increasing the rigidity of the racket frame, it has been found necessary to take into account, both the rigidity in the ball striking face inner direction and the ball striking face outer direction, instead of improving only one of such rigidities.
Meanwhile, with respect to the sweet area in the ball striking face, since the speed of the ball striking face in the striking direction becomes faster in the upper portion of the ball striking face than in the lower portion thereof during the the actual play (or during swing), i.e. in the forward end side (the top portion side) than in the hand-held side (the yoke side) of the racket frame in rotational speed, it is possible to transfer a larger kinetic energy to the ball by selecting a position of a striking point of the sweet area designed to provide the maximum repulsion coefficient, to be at a higher position in the ball striking face for striking the ball at said sweet area, thereby improving the repulsion performance of the racket frame.
In conventional racket frames in general, the sweet area is set at a position located somewhat at a lower side (yoke portion side) from the center of the ball striking face, by taking into account the facilitation of ball striking. As described above, since the sweet area where the repulsion coefficient becomes a maximum is set towards the side of the yoke portion from the center of the ball striking face, the rotational speed of the swing can not be effectively utilized, with a consequent deterioration in the repulsion coefficient.
As described so far, in tennis racket frames conventionally proposed or provided, there has been room for improvement from the viewpoint of the repulsion performance, with respect to the rigidity of the frame and the position of the sweet area, and it is expected that the flying performance of the ball may be improved by improving the above points.